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Microelectronic imaging devices and associated methods for attaching transmissive elementsRelated Patent Categories: Radiant Energy, Photocells; Circuits And Apparatus, Photocell Controlled Circuit, Plural Photosensitive Image Detecting Element ArraysMicroelectronic imaging devices and associated methods for attaching transmissive elements description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080001068, Microelectronic imaging devices and associated methods for attaching transmissive elements. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention is directed generally toward microelectronic imaging devices and associated methods for attaching transmissive elements, including methods for forming standoffs and attaching transmissive elements at the wafer level. BACKGROUND [0002] Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and personal digital assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts. [0003] Microelectronic imagers include image sensors that use Charge Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid-state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly "packaged" to protect their delicate components and to provide external electrical contacts. [0004] An image sensor generally includes an array of pixels arranged in a focal plane. Each pixel is a light sensitive element that includes a photogate, a photoconductor, or a photodiode with a doped region for accumulating a photo-generated charge. Microlenses and color filter arrays are commonly placed over imager pixels. The microlenses focus light onto the initial charge accumulation region of each pixel. The photons of light can also pass through a color filter array (CFA) after passing through the microlenses and before impinging upon the charge accumulation region. Conventional technology uses a single microlens with a polymer coating, which is patterned into squares or circles over corresponding pixels. The microlens is heated during manufacturing to shape and cure the microlens. Use of microlenses significantly improves the photosensitivity of the imaging device by collecting light from a large light-collecting area and focusing the light onto a small photosensitive area of the sensor. [0005] Manufacturing image sensors typically includes "post-processing" steps that occur after the microlens array is formed on a workpiece. Accordingly, it is necessary to protect the microlens array during these post-processing steps to prevent the microlens array from becoming contaminated with particles that might be released during these steps. One approach to addressing the foregoing manufacturing challenge is to attach individual image sensor dies to a substrate, tape over the corresponding sensor arrays, and then use a molding process to form "standoffs" to which a cover glass is mounted. The cover glass can accordingly protect the image sensor during subsequent processing steps, and becomes part of the sensor package. [0006] One drawback with this approach is that it is performed at the die level and accordingly cannot protect the sensor arrays during processing steps that occur before the dies have been singulated from a corresponding wafer or other larger workpiece. Another drawback with this approach is that a mold release agent is typically used to release the die from the mold machine in which the standoffs are formed. However, the mold release agent tends to inhibit the adhesion of adhesive compounds, which are required to attach the cover glass. Accordingly, the standoff surfaces must typically be cleaned (e.g., with a plasma process) before attaching the cover glass. This additional cleaning step increases the cost of manufacturing the die, and reduces manufacturing throughput. BRIEF DESCRIPTION OF THE DRAWINGS [0007] FIG. 1A illustrates a workpiece having multiple dies that may be processed and separated in accordance with an embodiment of the invention. [0008] FIG. 1B illustrates an imager device that includes a die singulated from the workpiece shown in FIG. 1A. [0009] FIGS. 2A-2B are flow diagrams illustrating methods for processing a workpiece in accordance with an embodiment of the invention. [0010] FIGS. 3A-3K illustrate a process for forming imager devices at the wafer level via a protective removable cover material and a single transmissive element. [0011] FIGS. 4A-4C illustrate a method for forming imager devices using multiple transmissive elements and a protective removable cover material in accordance with another embodiment of the invention. [0012] FIGS. 5A-5C illustrate a method for protecting sensitive portions of an imager wafer with a mold, and applying a single transmissive element to multiple dies in accordance with another embodiment of the invention. [0013] FIGS. 6A-6C illustrate a method for protecting sensitive portions of an imager wafer with a mold using multiple transmissive elements in accordance with still another embodiment of the invention. DETAILED DESCRIPTION [0014] The following disclosure describes several embodiments of imager workpieces and corresponding methods for manufacturing a plurality of microelectronic imaging units. A method for manufacturing a plurality of microelectronic imaging units in accordance with one aspect of the invention includes providing an imager workpiece having multiple image sensor dies configured to detect energy over a target frequency range, the image sensor dies having an image sensor and a corresponding lens device positioned proximate to the image sensors. The method can, in some embodiments, further include positioning standoffs adjacent to the lens devices while the image sensor dies are connected to each other via the imager workpiece. At least one transmissive element can be attached to the workpiece at least proximate to the standoffs so that the lens devices are positioned between the image sensors and the at least one transmissive element. Individual image sensor dies can then be separated from each other. [0015] In particular aspects of the invention, positioning the standoffs can include disposing portions of a removable cover material on the lens devices, positioning the imager workpiece in a mold, and forming the standoffs by introducing a flowable mold material into the mold and into regions between the portions of cover material. In another aspect of the invention, positioning the standoffs can include positioning the imager workpiece in a mold with cover portions of the mold positioned adjacent to the lens devices. The method can further include forming the standoffs by introducing a flowable mold material into the mold and into regions between the cover portions of the mold, while at least restricting contact between the mold material and the lens devices with the cover portions of the mold. [0016] An imager workpiece in accordance with another aspect of the invention can include a substrate having multiple image sensor dies. The image sensor dies can have image sensors configured to detect energy over a target frequency range, and corresponding multiple lens devices positioned proximate to the image sensors. The workpiece can further include at least one transmissive element attached to the imager workpiece so that the lens devices are positioned between the corresponding image sensors and the at least one transmissive element. The at least one transmissive element can be transmissive over at least part of the target frequency range. In one aspect of the invention, the at least one transmissive element can include multiple transmissive elements, with each transmissive element positioned adjacent to a corresponding image sensor die. In another aspect of the invention, the at least one transmissive element can include a single transmissive element positioned adjacent to multiple image sensor dies. [0017] Specific details of several embodiments of the invention are described below with reference to CMOS image sensors to provide a thorough understanding of these embodiments, but other embodiments can use CCD image sensors or other types of solid-state imaging devices. Several details describing the structures and/or processes that are well known and often associated with other types of microelectronic devices are not set forth in the following description for purposes of brevity. Moreover, although the following disclosure sets forth several embodiments of different aspects of the invention, several other embodiments of the invention can have different configurations or different components than those described below. Accordingly, the invention may have other embodiments with additional elements or without several of the elements described below with reference to FIGS. 1-6C. [0018] FIG. 1A illustrates a workpiece 102 carrying multiple dies (e.g., imager dies) 110. The workpiece 102 can be in the form of a wafer 101 or other substrate at which the dies 110 are positioned. Many processing steps can be completed on the dies 110 before the dies 110 are separated or singulated to form individual imaging devices. This approach can be more efficient than performing the steps on singulated dies 110 because the wafer 101 is generally easier to handle than are the singulated dies 110. As discussed in greater detail below, the dies 110 can include sensitive and/or delicate elements, and accordingly, it may be advantageous to protect these elements during the wafer-level processing steps. [0019] FIG. 1B illustrates a finished, singulated imaging device 100 after being processed in accordance with an embodiment of the invention. The imaging device 100 can include a die 110 singulated from the workpiece 102 described above with reference to FIG. 1A. The die 110 can include an image sensor 112, which can in turn include an array of pixels 113 arranged in a focal plane. In the illustrated embodiment, for example, the image sensor 112 can include a plurality of active pixels 113a arranged in a desired pattern, and at least one dark current pixel 113b located at a perimeter portion of the image sensor 112 to account for extraneous signals in the die 110 that might otherwise be attributed to a sensed image. In other embodiments, the arrangement of pixels 113 may be different. [0020] A color filter array (CFA) 114 is formed over the active pixels 113 of the image sensor 112. The CFA 114 has individual filters or filter elements 116 configured to allow the wavelengths of light corresponding to selected colors (e.g., red, green, or blue) to pass to each pixel 113. In the illustrated embodiment, for example, the CFA 114 is based on the RGB color model, and includes red filters, green filters, and blue filters arranged in a desired pattern over the corresponding active pixels 113a. The CFA 114 further includes a residual blue section 118 that extends outwardly from a perimeter portion of the image sensor 112. The residual blue section 118 helps prevent back reflection from the various components within the die 110. Continue reading about Microelectronic imaging devices and associated methods for attaching transmissive elements... 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